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Michael Levine, J. Jason McAnany, Jennifer Anderson; The effect of curvature on the grid illusions: Influence of a homunculus?. Journal of Vision 2008;8(6):278. doi: 10.1167/8.6.278.
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The intersections of lighter alleys defining a grid of black squares display illusory effects (the Hermann grid and the scintillating grid). We have noted that a single light disk in an intersection at some remove from fixation is rendered less visible (the “vanishing disk”). Furthermore, contrast threshold for the vanishing disk becomes even higher when the alleys are curved instead of straight (Levine & McAnany, VSS 2006). We speculated that the effect of curvature is due to either an attention shift or inhibition from a “higher” center than the detection mechanism (Levine & McAnany, in press). If this is correct, one would expect a measurable latency for the onset of this influence.
To test for this temporal disparity, we presented vanishing disk stimuli in which half of each presentation featured straight alleys, and half featured curved alleys. That is, in the middle of a 250 msec presentation of a stimulus, straight alleys abruptly curved, or vice versa. Since these changes are equivalent, any difference in threshold for the disk between these conditions must be caused by the order of onset of curvature, and not be simply an effect of a transition.
Thresholds for light disks were significantly higher when moderately curved alleys straightened than when straight alleys became curved to the same degree. We infer that the earlier curvature has time to initiate whatever disrupts visibility even though the alleys are straight in the latter part of the presentation; curvature later did not have time to exert its influence before detection was effected. (Note that had the decision been based on visual short-term memory, the stimuli that concluded with straight alleys would have evinced lower thresholds, not higher.) These and related results indicate a higher-level effect of curvature (complexity) upon detection.
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